1 - Get a ruler, it can be graduated in either centimetres or inches. Get a camera.
2 - Arouse yourself until you have a full erection.
3 - While standing, hold your erection parallel to the floor and gently pull your foreskin back if you are uncircumsized.
4 - Place the ruler on top of your erect penis and gently press the ruler into your pubic bone. This is called the “Bone Pressed Method” of penis measurement and it gives consistent results because there is a definite place to measure from.
5 - Take a photograph that clearly shows both ends of the ruler and your erect penis.